Core Graphics – Line Drawing Explained

I had to draw a dotted line today. Wow. It had to be round dots.

I found this snippet on StackOverflow.

CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat lengths[2];
lengths[0] = 0;
lengths[1] = dotDiameter * 2;
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineWidth(context, dotDiameter);
CGContextSetLineDash(context, 0.0f, lengths, 2);

I loved his comment to his code: “This should work correctly.”

The thing is, it didn’t quite. The dots didn’t look quite round, but sort of stretched. So I wanted to figure out why that is. (Answer, I never did, but thank you fudge factors and trial and error.) But I did set about visualizing these above parameters and have summarized it in a sketch:

So, perhaps that is of some help to you. Note, the image is displaying what you’d see if lengths were both dotDiameter.

In the end however, I just had to create some fudge factors to make it look right:

CGFloat dotDiameter = 6;
CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat lengths[2];
lengths[0] = 0.0001;  // Arguably FUDGE
lengths[1] = dotDiameter * 2.5f;  // FUDGE
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineWidth(context, dotDiameter * 1.2f);  // FUDGE
CGContextSetLineDash(context, 0.0f, lengths, 2);

Sometimes I’m a blacksmith programmer. Just keep pounding the metal until it has the right shape. This is one of those times. 🙂

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